The present invention relates to a method and apparatus for the compensation of spurious signals in a video signal.
With picture signal sources, for exampletelevision cameras and television film scanners comprising photoconductive tubes, semiconductor sensors or flying spot scanners, spurious interference signals exist in both the horizontal frequency region and in the vertical frequency region, superimposed on the useful picture signal partially in an additive manner and partially in a multiplicative manner. The additive spurious signals are produced by impurities in the target of the vidicon tubes and imperfections in the crystal of the semiconductor sensors. These imperfections are particularly visible in the dark portions of the picture and influence the dark current. Multiplicative spurious signals are largely produced by vignetting of the objective or illumination errors at the light source during film scanning, by imperfections in the crystal and burning-in of the target of vidicon tubes, by grain structure and burning-in of the flying-spot scanning tubes and by variations in sensitivity of the light sensitive regions of the semiconductor sensors. The multiplicative spurious signals are a function of the exposure time.
These interference signals may be expressed as follows: E(x, y)=k.sub.2 .multidot.f.sub.2 (x, y).multidot.I(x, y)+k.sub.1 .multidot.f.sub.1 (x, y), wherein E (x, y) represents the video signal as a function of the exposure location in x, y co-ordinates, I (x, y) represents the exposure at the co-ordinates x, y, k.sub.1 and k.sub.2 represent constants, f.sub.1 (x, y) represents the additive spurious signal at the co-ordinates x, y, f.sub.2 (x, y) represents the multiplicative spurious signal at the co-ordinates x, y.
Previously, it was customary to compensate a portion of the spurious signals by additive and multiplicative sawtooth and parabola functions. This method fails with complicated interference functions, for example burning-in of the target or of the flying-spot scanning tubes or with variations in sensitivity of the light sensitive regions of the semiconductor sensors.
An optimal solution of the interference problem is only provided by detecting the deviation of each individual picture element from the desired value, storing the said value and generating a correction signal improved to that extent. Consequently, the corrected signal would be expressed: ##EQU1##
That means that a division of the multiplicative components and a subtraction of the additive components would need to take place. However, in practice, it has not been possible to carry out the division of wide band analogue video signals with sufficient quality because modulation dependent frequency response errors and signal-to-noise ratio deterioration as well as temperature instability act in an interfering manner.
A system is known from U.S. Pat. No. 3,919,473 in which the video signal is digitalised and in the periods in which the video signal represents only spurious signals, the video signal is digitally stored in a memory in the line direction and in the frame direction. After digital to analogue (D/A) re-conversion, this signal is then subtracted from the input signal during a normal scanning period. However, only additive spurious signal components can be corrected with this method.
From the BBC Report 1976/4, especially FIG. 13 on page 11, it is further known to digitalise the video signal, then to convert to logarithms, to derive the correction signal therefrom, to store the latter and to subtract it from the video signal, to pass the thus produced signal to an anti-logarithmic stage and then to a D/A converter at the output from which the spurious signal compensated picture signal can be extracted. This method has the disadvantage that it is very expensive due to the logarithmic and anti-logarithmic conversions. A further disadvantage exists due to the fact that the video signal must also undergo logarithmic and anti-logarithmic conversion and must, therefore, be additionally deteriorated.